PROJECT SUMMARY This application is submitted in response to RFA-DA-19-002, Development of Medications to Prevent and Treat Opioid Use Disorders and Overdose (UG3/UH3). The application describes a highly integrated project aimed at developing novel Gpr151 antagonists to facilitate long-term abstinence in opioid-dependent individuals. Gpr151 is an orphan G-protein coupled receptor (GPCR) that is expressed almost exclusively in the medial habenula. In exciting new data, we demonstrate that Gpr151 co-localizes with ? opioid receptors in the medial habenula and that this orphan receptor regulates the inhibitory effects of opioids on habenular neurons. Moreover, we show that Gpr151 plays a critical role in regulating the motivational properties of opioid drugs such as morphine and oxycodone in mice. Specifically, we find mice with null mutation in Gpr151 (Gpr151-/- mice) are resistant to the stimulant and rewarding effects of these opioids and self-administer lower quantities of oxycodone. Here, we propose to leverage this new knowledge, in conjunction with our expertise in small molecule drug development, to identify and optimize novel Gpr151 antagonists for the treatment of opioid dependence in humans. As Gpr151 is an orphan receptor for which there are no known ligands, the lack of potent agonists can hamper the identification of novel antagonists. However, we have now developed robust cell-based assays (and appropriate counter-screens) to reliably monitor Gpr151 function and used these assays to identify novel Gpr151 agonists. During the UG3 phase of this application, we will optimize this series of agonists to increase their potency and selectivity for Gpr151 (12 months). Once optimized, we will use potent and selective agonists from this series to facilitate the identification of novel antagonists at Gpr151 derived from the same or related chemical series (primary strategy) or via high-throughput screening (backup strategy) (12 months). Identification of novel Gpr151 antagonists will trigger the transition to the UH3 phase of the project. During the UH3 phase, we will employ an iterative medicinal chemistry based on structure-activity relationships (SAR) to optimize the potency and selectivity of novel Gpr151 receptor antagonists. SAR will also be used to optimize these antagonists for drug metabolism and pharmacokinetics (DMPK) and brain penetration properties, and identify those that are least likely to have toxicity in humans. We will assess the effects of those with the most favorable drug-like physiochemical properties on electrophysiological responses of medial habenula to opioid drugs. In addition, we will assess the in vivo efficacy these novel antagonists using the intravenous oxycodone self-administration and reinstatement of extinguished responding procedures in wild-type and Gpr151-/- mice. This multidisciplinary research plan will capitalize on the unique relevant scientific and drug discovery expertise of our team of committed investigators to develop novel therapeutics to facilitate abstinence in opioid-dependent individuals.